Light-emitting elements including an organic compound as a luminous body, which have features such as thinness, lightness, high-speed response, and DC driving at low voltage, are expected to be applied to next-generation flat panel displays. In particular, display devices in which light-emitting elements are arranged in a matrix are considered to have advantages of a wide viewing angle and high visibility over conventional liquid crystal display devices.
It is said that the light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes with an EL layer including a luminous body provided therebetween, electrons injected from the cathode and holes injected from the anode are recombined in the light emission center of the EL layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons relax to the ground state. A singlet excited state and a triplet excited state are known as the excited states, and it is thought that light emission can be obtained through either of the excited states.
In order to improve element characteristics of such light-emitting elements, improvement of an element structure, development of a material, and the like have been actively carried out (see, for example, Patent Document 1).